1. Field of the Invention
The present invention relates to a piezoelectric ceramic transformer, which is operable in a high frequency band and more particularly to a piezoelectric ceramic transformer for an onboard power supply whose smaller size and lower noize are demanded.
2. Description of the Prior Art
An electromagnetic transformer has recently been used as a switching power source to make a power circuit for electronic equipment more compact and an increase in switching frequency has been demanded to provide a more compact switching power source. However, as the switching frequency is increased, the power loss due to hysteresis loss and eddy current loss of the magnetic material used in the electromagnetic transformer and the loss due to the skin effect of the conductor are sharply increased, resulting in a considerably low efficiency of the transformer. Accordingly, the upper limit of the practical frequency of the electromagnetic transformer was at most 500 kHz.
In contrast to the general electromagnetic transformer, a multilayer piezoelectric ceramic transformer has been used in a resonant mode and had a number of advantages as follows:
(1) The size of the transformer can be made smaller since the energy density at the same frequency is higher. PA1 (2) The transformer can be made nonflammable. PA1 (3) No noise due to electromagnetic induction is generated.
The structure of a Rosen type piezoelectric transformer which is a typical prior art piezoelectric transformer will be described with reference to FIG. 1 as follows. In a piezoelectric ceramic plate for outputting a voltage which is provided with electrodes on surfaces thereof, a reference numeral 11 denotes a low impedance drive portion of the piezoelectric transformer, which is provided with electrodes 13 and 14 on the upper and lower surfaces thereof, respectively. The drive portion 11 is polarized in a thickness direction as shown by an arrow in the drawing. A reference numeral 12 denotes a high impedance generating portion which is provided with an electrode 15 on the side end face thereof and the generating portion 12 is polarized in a longitudinal direction of the piezoelectric ceramic plate as represented by an arrow in the drawing. For example, in the case that it is desired to output a high voltage by inputting a low voltage, the piezoelectric transformer operates as follows. When a drive voltage is applied across the electrodes 13 and 14, length extensional vibration is excited by a piezoelectric unstiffened effect with an electromechanical coupling factor k.sub.31 and in the generating portion 12, the high voltage can be outputted from the electrode 15 due to piezoelectric stiffened effect at an electromechanical coupling factor k.sub.33.
On the other hand, in the case that it is desired to output a low voltage by inputting a high voltage, it is apparent that it will be sufficient to use the high impedance portion and the low impedance portion as input and output sides, respectively.
Besides such a Rosen type piezoelectric transformer, the other piezoelectric transformer as shown in FIGS. 2A and 2B using a radial extensional vibration mode of a disc has been known in the art. The operation principle of the piezoelectric transformer as shown in FIGS. 2A and 2B is identical with that of the Rosen type transformer. In the drawings, arrows denote the polarization direction; 21 denotes a low impedance portion; 22 denotes a high impedance portion; 23, 24 and 25 denote electrodes; 26 and 27 denote electrical terminals; and 28 denotes a ground. In such a piezoelectric transformer, the high impedance portion 22 of the piezoelectric stiffened effect is polarized in a length direction or a radial direction as shown by arrows in FIG. 2B. A d.c. voltage is necessary to perform a polarization treatment as is well known. For example, in case of PZT family piezoelectric ceramics, the intensity of the necessary polarization field is about 4 kV/mm. As the distance between the electrodes in the high impedance portion becomes longer, the polarization voltage increases. Accordingly, the piezoelectric transformer may be damaged due to the high polarization voltage when the polarization treatment is performed.
The operation frequency of the prior piezoelectric transformers described above remains at most 200 kHz since it uses length longitudinal resonation or fundamental radial extensional resonation.
As mentioned in the description of the prior art, the applicable frequency of the prior art piezoelectric transformer is limited to low frequencies of not more than 200 kHz. Since the prior art piezoelectric transformer can be used only in a low frequency band, a higher current cannot flow through the transformer. Accordingly, it is not suitable for a transformer for a power source. Further, it is necessary to apply a d.c. high voltage upon the transformer since polarization treatment is performed in a length or radial direction. Therefore, the piezoelectric transformer may be damaged on this polarization treatment. The Rosen type piezoelectric transformer is disadvantageous in that it has a low exciting efficiency since it can not help relying upon the coupling factor k.sub.31 of the piezoelectric unstiffened effect which is remarkably lower than the coupling factor k.sub.33 of the piezoelectric stiffened effect.